Liquid crystal displays (LCDs) of relatively small size have been commercially available for over two decades. Recent improvements have permitted development of large size, high resolution displays which are useful in notebook and desktop computers. Such LCD panels, particularly color LCD panels, are used for flat screen televisions, projection television systems and camcorder view finders, with many more applications anticipated in the future.
Such display panels may take two forms: passive matrix and active matrix liquid crystal displays (AMLCDs). Passive matrix displays employ transparent electrodes patterned in perpendicular striped arrays on facing glass plates. Red, green and blue color filters on the inner surface of one of the glass plates provide the full color display. The passive matrix display is thought to be easier to fabricate than AMLCDs, but much more limited in performance capabilities.
The fabrication of an active matrix liquid crystal display involves several steps. In one step, the front glass panel is prepared, which involves deposition of a color filter element onto a suitable substrate, such as glass. Color filter deposition typically involves depositing a black matrix pattern and three primary (red, green and blue) color patterns within the spaces outlined by the black matrix. The color elements are each typically about 70 to 100 microns in width by 200 to 300 microns in length for notebook computer applications, for example. The front glass substrate is completed by deposition of a transparent conducting layer over the color filter element. In a second step, a separate (rear) glass panel is used for the formation of thin film transistors or diodes, as well as metal interconnect lines. Each transistor acts as an on-off switch for an individual color pixel in the display panel. The third and final step is the assembly of the two panels, including injection of a liquid crystal material between the two panels to form the liquid crystal cell.
Ideally, in LCD displays, the transparent conducting layer, which typically is indium tin oxide (ITO), should be as smooth as possible to ensure electrical continuity. In addition, any thickness variations in the glass substrates or coatings can result in visible defects in the final display. Consequently, it is also important that the liquid crystal layer that fills the gap between the front and back panels be as uniform as possible across the entire display. Because the glass substrate which forms the front panel is itself a relatively flat article having parallel sides, any variations in thickness usually occur as a result of the process used to deposit the color filter array. It is therefore desirable to deposit color filter patterns which have a smooth upper surface and as uniform a thickness as is possible, because once a uniform thickness color filter/substrate composite has been obtained, it is a relatively straight forward process to deposit a smooth ITO layer and obtain a uniform cell gap when the front panel is combined with the rear panel. For this reason, photolithographic techniques are now preferred over printing techniques for forming color filters, because photolithography is capable of forming uniform color arrays. Nonetheless, all the deposition methods used thus far, including photolithography, by themselves have not been capable of depositing sufficiently smooth color patterns. Consequently, a planarizing layer is commonly applied over the color patterns to alleviate any imperfections in coating smoothness or thickness uniformity due to the deposition process. The transparent planarizing layer also serves to protect against ion migration to or from the ITO layer and color pattern layer. The planarizing layer should also be as smooth and flat as possible.
To facilitate deposition of a flat planarizing layer, it is desirable that the color patterns be as smooth, flat and substantially parallel to the undersurface of the glass substrate. Also, color patterns of uniform cross-section are desirable for obtaining optimum display contrast and color performance, because if the thickness of the pattern varies, the transmitted light intensity will vary.
One method heretofore used to form color filters is photolithography, in which each color pattern in the color filter is deposited in a separate step. As mentioned above, photolithography has, in the past, been a preferred method of depositing color filters, especially when compared to ink printing methods such as waterless lithography, gravure and typography, because photolithography can deposit image dots having a more flattened, rectangular cross-section, which is preferred. The printed ink dot, on the other hand, typically has a more round-topped or triangular cross-section due to surface tension effects. In addition, in typical printing processes, because the ink tends to wet both surfaces during a transfer from roll to roll or from roll to substrate, the inks tend to split cohesively to some extent during such transfers. This may further contribute to non-uniformity of the ink dot thickness, particularly for high viscosity inks. This results in an ink dot which, when deposited onto a substrate and cured, has a non-uniform cross-sectional shape, and this in turn results in an uneven surface which is more difficult to alleviate using a planarizing layer. In addition, photolithographic printing methods are inherently more accurately registered because the alignment between different color patterns is accomplished by optical rather than mechanical methods, and optical methods are intrinsically more precise. For all of these reasons, various prior workers in the flat panel display art have concluded that printing methods are substantially inferior for making color filters for LCD panels.
For example, the authors of "Color Filter for Liquid Crystal Display" by Ueyama et al , SEMI-SEMICON/West 92, International Flat Panel Display Conference, Section B, Pages 41-59, explain that, while printing methods are less expensive, the accuracy of ink printing methods is not sufficiently reliable to make high quality color filter components. The article points out, as also mentioned hereinabove, that printing methods are thought to be quite inferior in quality compared to photolithography, primarily because of the rounded cross-sectional shape of printed dots.
K. Mizuno and S. Okazaki, in The Japanese Journal Of Applied Physics, Vol. 30, No. 118, November, 1991, pp. 3313-3317, proposed producing a color filter by a process wherein ink patterns are successively prepared on a transfer (offset) roll and cured by exposure to ultraviolet light (UV) prior to transfer to the substrate. Each cured ink color pattern is individually transferred to a glass substrate coated with an adhesive layer.
U.S. Pat. No. 4,445,432 discloses a method and apparatus, relevant to a different art, for applying thermoplastic decorative inks onto various substrates by printing each color ink onto a releasing surface from a heated engraved or etched metal surface, transferring the various colors from each releasing surface onto a second releasing collector surface to form a multi-colored print, and transferring the multi-colored print to a ceramic, glass-ceramic or glass substrate. Various color inks are successively printed onto a collector roll, after which the resultant pattern is transferred to the substrate. Such processes have not been used to make color filter patterns.
U.S. Pat. No. 4,549,928 (Blanding et al.) describes using a similar technique for printing phosphors and a black matrix onto color TV panels. In this operation, thermoplastic pressure-sensitive inks, corresponding to the red, green and blue color phosphors and the black matrix, are applied separately to the collector roll to form the desired pattern. This pattern is then transferred to the TV tube panel.
Unfortunately, all of the techniques described above result in the ink dots having the conventional rounded or triangular cross section. It would be desirable to develop a method which results in smoother, more uniform ink dot shapes which are more suitable for color filter array applications.
In addition, color filter arrays typically undergo rather severe heating and treatment steps during manufacture of the LCD display. For example, the transparent conducting layer, typically indium tin oxide (ITO), is usually vacuum sputtered over the color filter array panel. This commonly takes place at temperatures elevated as high as 250.degree. C., for times which may be as long as one hour or more. Also, the liquid crystal is assembled by laminating the front and rear glass panels under pressure with thermally curable adhesives, which typically require temperatures in excess of 200.degree. C. Not all materials can withstand such high temperatures.
The printing techniques disclosed in the '432 and '928 patents employ pressure-sensitive hot-melt inks, which are printed from heated gravure rolls. The inks cool sufficiently on the offset surfaces to develop the cohesive strength necessary to achieve 100% ink transfer between the offset surfaces and the collector roll, and between the collector roll and the substrate. In some respects, hot-melt inks are less desirable than radiation curable inks. For example, slight temperature variations in the imaging or print transfer surfaces can result in registration variability. In addition, exposure to the 200.degree.-250.degree. C. temperatures inherent in the sputtering operations used to deposit the transparent electrode can cause conventional hot melt inks to undergo shape deformation, oxidative degradation, or volatilization.
It would be desirable to provide high quality, uniform thickness ink color filter arrays, having good resolution and registration, which can be obtained easily and at a lower cost than prior art color filter arrays. It would also be desirable to provide color filter array elements which can withstand the heating and treatment steps employed in making a liquid crystal display device.